The invention relates to a wound-rotor slip-recovery motor drive system. The wound-rotor slip-recovery drive has been recognized as very efficient, rugged and low cost to operate.
Static slip-recovery drives are known and have been found to be advantageous for many specific applications. Slip-recovery is a technique generally used with an induction motor of the wound-rotor type. When a variable voltage, variable frequency AC power supply is not available for the stator, or such commplexity is not desired, a variable speed drive can be achieved by controlling the rotor current. This approach which involves a controlled return to the network of the energy not used by the load, has been known with an additional machine as the Scherbius system, or as the Kramer system. With the advent of SCR power switches, control of the rotor current and slip-recovery have been practiced statically. See for instance "Principles of Inverter Circuits" by B. D. Bedford and R. G. Hoft, page 404, FIG. 1154, John Wiley 1964. See also: Proc. IEE, Vol. 110, No. 8, August 1963, "Switching Drive of Induction Motors" by M. S. Erlicki and Y. Wallach, pp. 1441-1450; IEEE transactions PAS-85, No. 1, January 1966, "Inverter Motor Speed Control With Static Inverters in the Rotor", pp. 76-84; and IEEE transactions IGA-5, No. 1, January/February, 1969, "Slip Power Recovery in an Induction Motor by the Use of a Thyristor Inverter" by William Shepherd and Jack Stanway, pp. 74-82.
The prior art method consists in rectifying the AC current induced as secondary in the rotor of the motor and in creating in the DC link a counter-electromagnetic-force voltage opposed to the rectified DC voltage, through coupling of an inverter between the AC power supply and the DC link. Generally, a transformer is used to couple the AC side of the inverter to the network. Thus, SCR control provides speed and torque control of a wound-rotor motor having a stator powered from a constant voltage-fixed frequency source.
The motor is controlled by controlling the gating angle of the thyristors of the inverter, therefore, the back EMF introduced in the DC link by the inverter, and ultimately the current in the rotor. In other words, the inverter controls the DC link voltage, whereby the DC link current is controlled, thus, the AC current in the wound-rotor. Accordingly, the torque is being controlled. The regulator of such a slip-recovery system includes two nested loops: a current inner loop and a speed outer loop.
Such motor drives are particularly useful for pump and fan drives, since these generally operate close to top speed. However, motor operation is not continuous and there is a speed cycling from zero speed to full speed and back. Such abrupt changes in and out of normal operation are extremely unfavorable in several respects.
It is known how to control the operation of a wound-rotor slip-recovery motor drive to achieve speed and torque control by controlling in a delayed ignition angle mode the thyristors of the inverter coupled between the DC link and the main power supply. See for instance:
Shepherd, W. and Stanway, J., "The Polyphase Induction Motor Controlled by Firing Angle Adjustment of Silicon Controlled Rectifiers", IEEE Interact Convention Record 1964, (4) pp. 135-154.
This type of drive, however, raises problems which are to be solved in order to take full advantage of these inherent qualities. For instance, outside the normal operative range, measures have to be taken for speed adjustment, either in order to smoothly and rapidly increase or reduce the speed or for an abrupt stop in case of an emergency. Moreover, there is a need to prevent an excessive motor rating normally called for operation outside the operative range.
In contrast to the aforementioned control approach, it is now proposed to substitute SCR devices for the diodes which constitute the rectifier side in the rotor of the wound-rotor motor, and to so control the SCR devices at the rectifier side, concurrently with the SCR devices at the power line side, so as to achieve similar and better results.
The idea of replacing the diodes of the rectifier bridge of the rotor of a slip-recovery system by SCR devices is found in "Control in Power Electronics and Electrical Drives" Proceedings of the Second IFAC Symposium, Dusseldorf, West Germany, Oct. 3-5, 1977 (Program Press 1978) pp. 559-566 in a paper entitled "Supersynchronous Static Converter Cascade" by P. Zimmermann. The object, in this prior art, is to extend the operative range of a wound-rotor slip recovery drive above synchronous speed, in fact doubling for the same rating, the speed range so that the SCR's are working as diodes in the rectifier bridge within the subsynchronous range and as inverter devices of the same bridge when in the supersynchronous range. Moreover, in the aforementioned paper of Zimmermann, the rotor bridge while in the inverter mode is controlled by forced commutation.
An object of the present invention is to maximize the efficiency of a wound-rotor slip-recovery drive for speeds below the normal speed operative range.
It is also an object of the present invention to permit quickly extinguishing the rotor current of a wound-rotor slip-recovery motor drive in case of an emergency.
Still another object of the present invention is to reduce the overall rating requirement of a wound-rotor slip-recovery system.
These and other objects will appear from the description hereinafter of the invention in its preferred embodiments.